Posts Tagged ‘Ideas’

New Class of Polymers which Allow Efficient Recycling

Friday, May 10th, 2019

In the news I saw a reference to this paper on recyclable polymers1. The basic idea described is that these researchers have developed a class of polymers which allow efficient recycling. By combining the monomers of a bis-triketone and a di-amine a polymer is formed. This polymer can be broken back down to the monomers simply by soaking in acid.

The important advancement here is that once the starting monomers are regenerated they can be purified from any additives, and cleaned from impurities from the use of the polymer, and then used as starting materials for the generation of new polymer. This allows the recycled product to be just as high quality as the original product.

Figure 1. A triketone

Figure 1. A triketone

Let's take a look at the chemical reactions going on here. First, when they say they are using a triketone it is specifically a molecule with a carbon surrounded by three carbonyl moieties, an example is shown in Figure 1. A triketone is more appropriate than a normal ketone for this sort of reaction because it has a fairly acidic proton on that central carbon, the acidity is caused by stabilization of the resulting anion by the surrounding carbonyl groups, which can be represented using resonance structures as shown in Figure 2.

Figure 2. Resonance structure of a triketone anion.

Figure 2. Resonance structure of a triketone anion.

The reaction creating the polymer is the reaction of a triketone with an amine, which forms a diketoimine and releases a molecule of water, as shown in Figure 3.

Figure 3. Formation of a diketoimine.

Figure 3. Formation of a diketoimine.

This reaction can be reversed by hydrolysis under acidic conditions to regenerate the original triketone and amine, as shown in Figure 4.

Figure 4. Hydrolysis of diketoimine.

Figure 4. Hydrolysis of diketoimine.

The formation of a polymer would use a diamine and a bis-triketone as starting materials. When mixed, they form a polymer, as shown in Figure 5. Here I have shown the reaction using 1,2-diethylamine and the bis-triketone derived from adipic acid.

Figure 5. Polymerization

Figure 5. Polymerization

Figure 6. A triamine.

Figure 6. A triamine.

The reaction shown above would result in in linear polymer. A branching or networked polymer can be created using a triamine, such as the one shown in Figure 6.

Figure 7. A bis-triketone derived from terephthalic acid.

Figure 7. A bis-triketone derived from terephthalic acid.

The physical properties of the resulting polymer can be controlled by the choice of monomers. A mixture of diamine and triamine monomers could be used to tailor the polymer for a specific use. Instead of using the aliphatic bis-triketone derived from adipic acid, one could start with the aromatic bis-triketone derived from terephthalic acid, which is shown in Figure 7. This would give more rigidity to the resulting polymer. Alternately, one could form a polymer with less rigidity by using a bis-triketone derived from a longer chained di-acid, such as dodecanedioic acid.

The real advantage this polymer presents is that additives such as colorants, platicizers, or inorganic fibers for strength can be added without reducing the recylability of the polymer. Once the polymer is broken down by acid, the monomers can be purified using regular solution purification techniques.

I also wanted to call out at the formation of the bis-triketone, because I think the chemistry here, while not new in itself, is interesting as an example of organic synthesis. They start with two equivalents of dimedone2 and one equvalent of adipic acid3. These were dissolved in methylene chloride with three equivalents of DMAP4, which will deprotonate both the adipic acid and the dimedone. They then added a solution of DCC5 in methylene chloride, which accepts the oxygen from the adipic acid, forming N,N'-dicyclohexylurea and allowing the coupling of the adipic acid and dimedone. The N,N'-dicyclohexylurea precipitates and was removed by filtration, and then the DMAP was removed by washing with hydrochloric acid. This scheme is shown below in Figure 86.

Figure 8. Reaction to form a bis-triketone.

Figure 8. Reaction to form a bis-triketone.

  1. Peter R. Christensen, Angelique M. Scheuermann, Kathryn E. Loeffler, Brett A. Helms, Closed-loop recycling of plastics enabled by dynamic covalent diketoenamine bonds, Nature Chemistry, volume 11, pages 442–448 (2019). []
  2. 5,5-Dimethyl-1,3-cyclohexanedione []
  3. Hexanedioic acid. You need 2 eq. of dimedone because the reaction adds one dimedone to each end of the adipic acid. []
  4. Dimethylaminopyridine, a base. You might think this would require four equivalents, but the DCC acts as a base as well so you don't need so much []
  5. Dicyclohexylcarbodiimine []
  6. I admit that I am being lazy about arrow pushing electrons here, but I think this shows the important part of the reaction. []

Ideas

Wednesday, May 1st, 2019

Have you ever had a great idea and then later realized that you were not the first one to think of it?

Asphalt paver machine

Asphalt paver machine

When I was about four I invented a machine that would make roads. I even drew a really nice picture in crayons of what I came up with. It had a big hopper in front, and would pour the material down and spread it flat and it had big treads on the side so it could move forward building a road behind it. Some years later I was amazed as my family drove past a highway under construction, and they were using the machine that I had drawn! Something like the picture shown.

When I was in college, after studying a bit of chemistry and physics, I did some thinking about nuclear fusion. Some metals, such as palladium, can absorb large amounts of hydrogen. I reasoned that if one was to make a target of palladium and expose it to deuterium it would hold the deuterium in place. One could then use some high voltage to accelerate more deuterium ions toward it, resulting in nuclear fusion. I was thinking this would be a great way to make a power-producing fusion reactor! Then I found out that this had already been invented some decades prior. The resulting nuclear reactors are not used to generate power, but they are used in the medical field as a source of high energy neutrons.

Also while in college I gave some thought to unusual electric motor configurations. I was fascinated by the homopolar motor. I drew up an idea of a homopolar motor made with multiple windings around two co-axial magnets which were arranged with their north poles pointed in opposite directions. Some years later I found some of my old notes and drawings, and so I did some searching on the internet to try to decide if there was any way they would work. I was surprised to find that a patent had been issued in the intervening years for such a device1, and the drawings in the patent are very similar to what I had drawn some years before.

  1. Although, there being a patent does not necessarily mean that it would actually work. []